The present invention relates to a voltage-controllable, variable capacitance element, more particularly intended for integration on to an integrated circuit pellet. This controllable capacitance element makes it possible to eliminate shock chokes which filter the control voltage relative to the voltage of the signal, as well as the insulating capacitors normally separating a controllable capacitor according to the prior art and the remainder of the circuit. However, in the element according to the invention, the control voltage does not interfere with the signal. The controllable capacitance element according to the invention more particularly relates to the field of ultra-high frequencies.
Hitherto the best known and most widely used variable capacitance, not constituted by a capacitor whose value is mechanically modified, is a reverse biased Schottky diode, known as a varactor. The capacitance of the metallization on the semiconductor material of a Schottky diode varies with the control voltage applied to the diode cathode. This variable capacitor can be used as a tuning element for the oscillator or filter, but the disadvantage of such a variable capacitor is that the d.c. control voltage must be separated from the ultra-high frequency signal by a filter, having at least one and often two shock chokes. The presence of a shock choke on the surface of an integrated chip is to be avoided wherever this is possible, because a choke or filter occupies too much space on the chip and considerably decreases the integration density.
The variable capacitance element according to the invention is based on a well-known, voltage controllable, variable resistance element, namely the field effect transistor. In a non-polarized field effect transistor, the d.c. voltage applied to the gate varies the resistances between the source and the channel and between the channel and the drain, e.g. gripping the channel to a greater or lesser extent by the depletion zone in the case of a depletion field effect transistor. Thus, the control voltage on the gate of a field effect transistor is not directly connected to the signal voltage, so that a shock choke or filter is not necessary on the gate voltage. The control voltage acts by capacitive effect on the channel.
The equivalent circuit diagram of a variable capacitance element according to the invention switches over the resistors and capacitors of the equivalent circuit diagram of a field effect transistor, so that a T-structure diagram is obtained. The two terminals for access to said element are constituted by capacitors, either in the form of Schottky diodes, or junction diodes, the diodes being connected as a common cathode. The control terminal is constituted by a resistor, across which the control voltage is applied. The resistor is connected to the common point between the two diodes. In the case of the variable capacitance element construction according to the invention, the capacitances of the two diodes are pure capacitances, without a parasitic access resistance. According to a preferred embodiment of the invention, the diodes are constituted by a doped active zone in a semiinsulating substrate. Two Schottky metallizations are juxtaposed on said active zone, so as to form Schottky junctions. These two metallizations are in electrical contact with two microstrips forming the access terminals to the element. The doped zone also has a strip-like projection, on which there is an ohmic contact. The dimensions and doping level of said projection regulate the value of its resistance. The control voltage is applied to the ohmic contact.
In other embodiments adapted to the requirements, the variable capacitance element only has a single diode, or has more than two diodes, so that an element is obtained which, as from a single control voltage, makes it possible to control or tune a plurality of circuits.